Polyimide separator for semi-solid lithium batteries and method for preparing the same

Abstract

The application discloses a kind of polyimide separators for semi-solid lithium battery and preparation method thereof, belong to lithium battery separator technical field.The separator includes polypropylene base film and the gel polyimide coating layer being arranged in the both sides of polypropylene base film, the gel polyimide coating layer is made by fast ion conductor modified polyimide slurry spraying;The fast ion conductor modified polyimide slurry, by weight fraction, includes the following components: thermoplastic polyimide 3.5-4.5 parts;Bistrifluoromethanesulfonimide lithium 1-3 parts;Fast ion conductor lithium lanthanum zirconium oxygen 0.8-1.2 parts;Sulfolane 35-45 parts.The application utilizes thermoplastic polyimide with specific structure, introduces fast ion conductor lithium lanthanum zirconium oxygen and bistrifluoromethanesulfonimide lithium into it, increases the ionic conductivity of electrolyte.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium battery separators, and particularly relates to a polyimide separator for a semi-solid lithium battery and a preparation method thereof. BACKGROUND

[0002] As a recognized ideal energy storage element, lithium ion batteries have made great progress in the past few decades and have made outstanding contributions in the fields of portable electronic products and large-scale energy storage such as electric vehicles. With the large-scale application of lithium ion batteries, the safety and high energy density of the batteries become increasingly important and challenging. As a physical barrier for isolating the positive and negative electrodes of the battery and containing the electrolyte for ion shuttling, the separator needs to have the following characteristics for the safety and high energy density of the battery: light and thin, no obvious shrinkage at high temperature, good mechanical strength, and good affinity with the electrolyte.

[0003] Although the traditional liquid lithium battery has high ionic conductivity, the liquid electrolyte has a low boiling point and is easy to leak, which can easily cause lithium dendrite growth, has a relatively low energy density, and can easily cause safety problems. Solid-state electrolyte provides a good opportunity to solve safety problems and prevent lithium dendrite growth, but its interface stability is poor, the interface impedance is high, the production process is complex, and the manufacturing cost is high, so it is currently difficult to be applied on a large scale for commercialization. The performance of the semi-solid battery is better than that of the liquid battery but worse than that of the full solid battery, and the semi-solid battery has good safety, high energy density, long cycle life, a wider working temperature range, and good resistance to extrusion and vibration.

[0004] At present, the semi-solid battery separator mainly uses a wet polyolefin film coated with ceramic material, and the coating of the ceramic material improves the heat resistance of the separator, enhances the puncture resistance of the separator, and improves the safety of the battery in use. However, there are defects such as a large interface contact impedance between the battery electrode and the separator and a relatively low lithium ion conductivity, which need to be further improved. SUMMARY

[0005] One of the purposes of the present application is to provide a polyimide separator for a semi-solid lithium battery to solve the problem of a relatively low lithium ion conductivity.

[0006] The second purpose of the present application is to provide a preparation method of a polyimide separator for a semi-solid lithium battery.

[0007] The purposes of the present application can be achieved by the following technical solutions.

[0008] In a first aspect, a polyimide separator for a semi-solid lithium battery includes a polypropylene-based film and a gel polyimide coating layer arranged on both sides of the polypropylene-based film, and the gel polyimide coating layer is made of a fast ion conductor modified polyimide slurry.

[0009] The fast ion conductor modified polyimide slurry comprises the following components in parts by weight:

[0010] thermoplastic polyimide 3.5-4.5 parts;

[0011] lithium bistrifluoromethanesulfonimide 1-3 parts;

[0012] fast ion conductor lithium lanthanum zirconium oxide 0.8-1.2 parts;

[0013] sulfolane 35-45 parts.

[0014] Further, the polypropylene-based film has a melt flow rate (MI) of 0.3-0.5 g / 10 min, a tensile strength of 200-240 MPa, a pore size of 50-110 nm, and a porosity of 50-55%.

[0015] Further, the thermoplastic polyimide has a chemical structural formula as shown in Formula I:

[0016]

[0017] wherein x = 60-80%;

[0018] wherein R is any one of , n is any integer between 0 and 5, and R1 and R2 are -H or -CH3.

[0019] In a second aspect, a preparation method of a polyimide separator for semi-solid lithium batteries comprises the following steps:

[0020] Step 1, preparation of a fast ion conductor modified polyimide slurry: under stirring, thermoplastic polyimide is added to sulfolane, and after reaction at 140-160°C for 1-3 h, lithium bistrifluoromethanesulfonimide and fast ion conductor lithium lanthanum zirconium oxide are sequentially added, and the reaction is continued for 1-3 h to obtain a fast ion conductor modified polyimide slurry;

[0021] Step 2, corona treatment of a polypropylene-based film;

[0022] Step 3, spraying the fast ion conductor modified polyimide slurry on the surface of the corona-treated polypropylene-based film, and vacuum drying to obtain a gel semi-solid lithium battery polyimide separator.

[0023] Further, the preparation method of the thermoplastic polyimide comprises the following steps:

[0024] S1. Preparation of an amino-terminated polyamic acid prepolymer A:

[0025] Under nitrogen protection and at a temperature of 5°C or less, 4,4'-diaminodiphenyl ether and pyromellitic anhydride are sequentially added to N,N-dimethylacetamide to obtain an oligomerization degree low amino-terminated polyamic acid prepolymer A, which has a chemical structural formula of Formula II:

[0026]

[0027] Further, the molar ratio of anhydride in pyromellitic anhydride to amino group in 4,4'-diaminodiphenyl ether is 1:(1.2-1.5), and the solid content in the reaction system is 25-35%.

[0028] S2. Preparation of an acid anhydride-terminated polyamic acid prepolymer B:

[0029] Under nitrogen protection and at a temperature of 5°C or less, 4,4'-diaminodiphenyl ether and a diphenyl glycoside compound are sequentially added to N,N-dimethylacetamide to obtain an oligomerization degree low acid anhydride-terminated polyamic acid prepolymer, which has a chemical structural formula of Formula III:

[0030]

[0031]

[0032] Further, the diphenyl glycoside compound has a chemical structural formula of Formula IV:

[0033] wherein R is any one of R1 and R2 is -H or -CH3.

[0034] Further, the molar ratio of anhydride in the diphenyl glycoside compound to amino group in 4,4'-diaminodiphenyl ether is 1:(1.2-1.5), and the solid content in the reaction system is 25-35%.

[0035] S3. Preparation of a polyamic acid solution:

[0036] The amino-terminated polyamic acid prepolymer A and the acid anhydride-terminated polyamic acid prepolymer B are mixed uniformly and subjected to a condensation reaction to obtain a polyamic acid solution, and the polyamic acid has a chemical structural formula of Formula V:

[0037]

[0038] S4. Preparation of a thermoplastic polyimide:

[0039] The imidization reagent is added to the polyamide acid solution under stirring at room temperature (25-30 DEG C), and after mixing, the reaction is continued at 35-45 DEG C for 15-20 h to obtain a polyimide solution, which is cooled to room temperature, precipitated by adding ethanol, and vacuum dried to obtain the thermoplastic polyimide.

[0040] Further, the imidization reagent is composed of acetic anhydride and triethylamine in a molar ratio of 1:1.

[0041] Further, the molar ratio of the total amount of the imidization reagent to 4,4'-oxydianiline is 1:(0.2-0.8).

[0042] Compared with the prior art, the present application has the following advantages:

[0043] 1. The present application utilizes the thermoplastic polyimide with a specific structure, introduces the fast ion conductor lithium lanthanum zirconium oxide (LLZO) and bis-trifluoromethanesulfonimide lithium into the thermoplastic polyimide, increases the ionic conductivity of the electrolyte, improves the charging speed of the lithium battery, hinders the growth of lithium dendrites, and prepares a slurry to be sprayed on the surface of a dry-stretching polypropylene porous membrane base film to obtain a gel polyimide composite diaphragm with excellent comprehensive performance.

[0044] 2. In the present application, the obtained thermoplastic polyimide has a long-term heat resistance of 240 DEG C and can quickly form a gel in a typical lithium ion battery liquid electrolyte; the gel polyimide coating layer can greatly improve the heat resistance of the diaphragm and improve the safety of the lithium battery, and the gel polyimide is in close contact with the electrode, greatly reducing the interface impedance, reducing the amount of liquid electrolyte added during the preparation of the lithium battery, and meeting the preparation requirements of the semi-solid battery.

[0045] 3. In the present application, the diphenyl disaccharide compound is used to replace part of the pyromellitic anhydride to introduce ketone, ether, methylene chain and other structures into the molecular chain of the polyimide, increase the flexibility of the molecular chain, and improve the adhesion and toughness of the polyimide. The molecular structure is designed, the types (anhydride or amino) and contents of the end groups are controlled, the polyamide acid prepolymer with amino groups at the end and the polyamide acid prepolymer with anhydride at the end (B1, B2, B3, etc.) are synthesized, the A and B substances are added step by step by means of the step-by-step polymerization strategy, the molar ratio of the A and B substances and the polymerization reaction conditions are controlled, and the polyamide acid with different structures and molecular weights is obtained. The imidization reagent composed of dehydrating agent acetic anhydride and catalyst triethylamine is slowly added to the obtained polyamide acid solution to perform chemical imidization to obtain thermoplastic polyimides with different structures and molecular weights. BRIEF DESCRIPTION OF DRAWINGS

[0046] The present application will be further described below in conjunction with the drawings.

[0047] Figure 1 is a schematic diagram of the preparation process of the thermoplastic polyimide of the present application. DETAILED DESCRIPTION

[0048] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0049] The preparation flowchart of the thermoplastic polyimide in the following examples and comparative examples is shown in Figure 1

[0050] Example 1

[0051] A thermoplastic polyimide, the chemical structural formula of which is formula VI:

[0052]

[0053] wherein x = 75%.

[0054] The preparation method of the thermoplastic polyimide comprises the following steps:

[0055] S1. Preparation of an amino-terminated polyamic acid prepolymer A:

[0056] Under nitrogen protection and at a temperature of ≤5°C, 4,4'-diamino diphenyl ether is added to DMA as a solvent, and after complete dissolution, pyromellitic anhydride is added. The molar ratio of anhydride in pyromellitic anhydride to amino groups in 4,4'-diamino diphenyl ether is 1:1.4, and the solid content in the reaction system is 30%. After 4h of reaction, an amino-terminated polyamic acid prepolymer A with low degree of polymerization is obtained.

[0057] S2. Preparation of an anhydride-terminated polyamic acid prepolymer B:

[0058] Under nitrogen protection and at a temperature of ≤5°C, 4,4'-diamino diphenyl ether is added to DMA as a solvent, and after complete dissolution, 4,4'-oxybisphthalic anhydride is added. The molar ratio of anhydride in 4,4'-oxybisphthalic anhydride to amino groups in 4,4'-diamino diphenyl ether is 1:1.4, and the solid content in the reaction system is 30%. After reaction, an anhydride-terminated polyamic acid prepolymer B with low degree of polymerization is obtained.

[0059] S3. Preparation of a polyamic acid solution:

[0060] The amino-terminated polyamic acid prepolymer A and the anhydride-terminated polyamic acid prepolymer B are mixed uniformly in a certain proportion, and a condensation reaction is performed to obtain a polyamic acid solution.

[0061] S4. Preparation of a thermoplastic polyimide:​

[0062] Under stirring conditions at room temperature and 200 rpm, an imidization reagent (composed of acetic anhydride and triethylamine mixed in a molar ratio of 1:1) was added to the polyamic acid solution, and the molar ratio of the imidization reagent to the total amount of 4,4'-diaminodiphenyl ether was 1:0.5. After mixing evenly, the temperature was raised to 40°C and the reaction was continued for 18 hours to obtain a polyimide solution. The solution was cooled to room temperature, ethanol was added for precipitation, the precipitate was collected, and vacuum dried to obtain thermoplastic polyimide.

[0063] Examples 2-4

[0064] The chemical structural formulas of the thermoplastic polyimides in Examples 2-4 are Formula VII, Formula VIII, and Formula IX, respectively, as shown in Table 1;

[0065] The preparation method of thermoplastic polyimide described in Examples 2-4 is different from that in Example 1 in that the diphenyl dianhydride compound in S2 is 3,3',4.4'-tetrabenzophenone tetracarboxylic dianhydride. 2,2-Diphenylpropane-3,4,3',4'-tetracarboxylic dianhydride 3,3',4.4'-Biphenyltetracarboxylic acid The remaining steps and parameters remain the same.

[0066] Table 1

[0067]

[0068]

[0069] Example 5

[0070] A method for preparing a polyimide separator for a semi-solid lithium battery comprises the following steps:

[0071] Step 1, preparing a fast ion conductor modified polyimide slurry: adding 4 parts of the thermoplastic polyimide prepared in Example 1 to 40 parts of sulfolane with stirring at 200 rpm, reacting at 150° C. for 2 h, then adding 2 parts of lithium bis(trifluoromethanesulfonyl)imide and 1 part of fast ion conductor lithium lanthanum zirconium oxide in sequence, and continuing the reaction for 2 h to obtain a fast ion conductor modified polyimide slurry;

[0072] Step 2: using a polypropylene base film with MI = 0.4 g / 10 min, a tensile strength of 220 MPa, a pore size of 80 nm, and a porosity of 52%, and performing corona treatment on its surface;

[0073] Step 3: spray the fast ion conductor modified polyimide slurry on the surface of the polypropylene base film after corona treatment, and vacuum dry it to obtain a polyimide separator for a gel semi-solid lithium battery.

[0074] Example 6

[0075] A preparation method of a polyimide separator for semi-solid lithium batteries, which is different from Example 5 in that a thermoplastic polyimide prepared in Example 2 is used in Step 1, and the remaining steps and parameters remain unchanged.

[0076] Example 7

[0077] A preparation method of a polyimide separator for semi-solid lithium batteries, which is different from Example 5 in that a thermoplastic polyimide prepared in Example 3 is used in Step 1, and the remaining steps and parameters remain unchanged.

[0078] Example 8

[0079] A preparation method of a polyimide separator for semi-solid lithium batteries, which is different from Example 5 in that a thermoplastic polyimide prepared in Example 4 is used in Step 1, and the remaining steps and parameters remain unchanged.

[0080] Comparative Example 1

[0081] A preparation method of a polyimide, which is different from Example 1 in that the 4,4'-oxydiphthalic anhydride in S2 is replaced with an equal amount of pyromellitic anhydride, and the remaining steps and parameters remain unchanged.

[0082] Comparative Example 2

[0083] A preparation method of a polyimide, which is different from Example 1 in that the pyromellitic anhydride in S1 is replaced with an equal amount of 4,4'-oxydiphthalic anhydride, and the remaining steps and parameters remain unchanged.

[0084] Comparative Example 3

[0085] A preparation method of a polyimide separator for lithium batteries, which is different from Example 5 in that the polyimide prepared in Comparative Example 1 is used in Step 1, and the remaining steps and parameters remain unchanged.

[0086] Comparative Example 4

[0087] A preparation method of a polyimide separator for lithium batteries, which is different from Example 5 in that the polyimide prepared in Comparative Example 2 is used in Step 1, and the remaining steps and parameters remain unchanged.

[0088] Comparative Example 5

[0089] A preparation method of a polyimide separator for semi-solid lithium batteries, which is different from Example 5 in that no fast ion conductor lithium lanthanum zirconium oxide is added in Step 1, and the remaining steps and parameters remain unchanged.

[0090] The polyimide separator for lithium battery prepared in Example 5-Example 8 and Comparative Example 3-Comparative Example 5 was subjected to performance test, and the test results are shown in Table 2 and Table 3:

[0091] Table 2

[0092] Thickness (pm) Porosity (%) Air permeability (s / 100 mL) Puncture strength (gf) Example 5 12.0 55 120 275 Example 6 12.5 52 134 284 Example 7 11.8 56 107 281 Example 8 12.6 59 94 268 Comparative Example 3 12.1 52 78 231 Comparative Example 4 12.2 51 77 227 Comparative Example 5 12.4 53 118 262

[0093] Table 3

[0094]

[0095] The separator prepared in Example 5-Example 8 and Comparative Example 3-Comparative Example 5 was cut into a size matching with the 40mm*60mm metal plate, respectively, to assemble into a soft package battery, inject electrolyte, test alternating current impedance, then sequentially put in 1-4 layers of separator, test resistance value, take the number of separator layers as abscissa and the resistance of separator as ordinate to draw a curve, and the slope of the curve is the ionic conductivity, which can represent the migration ability of lithium ion in the separator, and the test results are shown in Table 4:

[0096] Table 4

[0097] Ionic conductivity (mS / cm) Example 5 1.84 Example 6 1.79 Example 7 1.66 Example 8 1.67 Comparative Example 3 1.49 Comparative Example 4 1.41 Comparative Example 5 1.27

[0098] As shown in Table 2-Table 4, the polyimide separator for semi-solid lithium battery prepared in the application has a thickness of 12±1μm, a porosity of 55±5%, a gas permeability of 120±40s / 100mL, a puncture strength of ≥200gf, a tensile strength MD of ≥200Mpa, a tensile strength TD of ≥15Mpa, a tensile elongation at break MD of ≥50%, a tensile elongation at break TD of ≥200%, a heat shrinkage rate (120℃ / 1h) MD of ≤1%, and a heat shrinkage rate (120℃ / 1h) TD of ≤0.5%, and has excellent performance and high ionic conductivity.

[0099] It should be noted that the relational terms herein such as first and second, and the like are used only to differentiate one entity or operation from another entity or operation, without necessarily requiring or implying any such actual relationship or order between such entities or operations. Moreover, the terms "comprising", "including", or any other variant thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can also include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0100] Although the embodiments of the present application have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, alternatives, and variations can be made thereto without departing from the principles and spirit of the present application, and the scope of the present application is defined by the appended claims and their equivalents.

Claims

1. A polyimide separator for a semi-solid lithium battery, characterized in that: It includes a polypropylene base film and a gel polyimide coating layer arranged on both sides of the polypropylene base film, wherein the gel polyimide coating layer is made by spraying a fast ion conductor modified polyimide slurry; The fast ion conductor modified polyimide slurry comprises the following components in parts by weight: 3.5-4.5 parts of thermoplastic polyimide; 1-3 parts of lithium bis(trifluoromethanesulfonyl)imide; Fast ion conductor lithium lanthanum zirconium oxide 0.8-1.2 parts; 35-45 parts of sulfolane; The chemical structural formula of the thermoplastic polyimide is Formula I: Formula I; Where x = 60-80%; Where R is 、 、 Any one of the following, wherein n is any integer between 0 and 5, and R1 and R2 are -H or -CH3.

2. A polyimide separator for a semi-solid lithium battery according to claim 1, characterized in that: The polypropylene-based film has a melt flow rate of 0.3-0.5 g / 10 min, a tensile strength of 200-240 MPa, a pore size of 50-110 nm, and a porosity of 50-55%.

3. A method for preparing a polyimide separator for a semi-solid lithium battery according to any one of claims 1 to 2, characterized in that: The following steps are involved: Step 1: Add thermoplastic polyimide to sulfolane under stirring, react at 140-160° C. for 1-3 hours, then add lithium bis(trifluoromethanesulfonyl)imide and fast ion conductor lithium lanthanum zirconium oxide in sequence, and continue to react for 1-3 hours to obtain a fast ion conductor modified polyimide slurry; Step 2, performing corona treatment on the surface of the polypropylene base film; Step 3: spray the fast ion conductor modified polyimide slurry on the surface of the polypropylene base film after corona treatment, and vacuum dry it to obtain a polyimide separator for a gel semi-solid lithium battery.

4. The method for preparing a polyimide separator for a semi-solid lithium battery according to claim 3, characterized in that: The preparation method of the thermoplastic polyimide comprises the following steps: S1. Adding 4,4'-diaminodiphenyl ether and pyromellitic anhydride to N,N-dimethylacetamide in sequence at ≤5°C under nitrogen protection to obtain an amino-terminated polyamic acid prepolymer A having a low degree of polymerization; S2. adding 4,4'-diaminodiphenyl ether and diphenyl diglycoside compound to N,N-dimethylacetamide in sequence under nitrogen protection and ≤5°C to react to obtain a low polymerization degree of anhydride-terminated polyamic acid prepolymer; S3. The amino-terminated polyamic acid prepolymer A and the anhydride-terminated polyamic acid prepolymer B are uniformly mixed and subjected to a condensation reaction to obtain a polyamic acid solution; S4. Add an imidization reagent to the polyamic acid solution under stirring at room temperature, mix well, raise the temperature to 35-45°C and continue the reaction for 15-20 hours to obtain a polyimide solution, cool to room temperature, add ethanol for precipitation, collect the precipitate, and vacuum dry to obtain a thermoplastic polyimide.

5. The method for preparing a polyimide separator for a semi-solid lithium battery according to claim 4, characterized in that: The molar ratio of the anhydride in pyromellitic anhydride to the amino group in 4,4'-diaminodiphenyl ether is 1:(1.2-1.5), and the solid content in the reaction system is 25-35%.

6. The method for preparing a polyimide separator for a semi-solid lithium battery according to claim 4, characterized in that: The chemical structural formula of the diphenyl diglycoside compound is Formula IV: Formula IV, wherein R is 、 、 Any one of the following, wherein n is any integer between 0 and 5, and R1 and R2 are -H or -CH3.

7. The method for preparing a polyimide separator for a semi-solid lithium battery according to claim 4, characterized in that: The molar ratio of the acid anhydride in the diphenyl diglycoside compound to the amino group in 4,4'-diaminodiphenyl ether is 1:(1.2-1.5), and the solid content in the reaction system is 25-35%.

8. The method for preparing a polyimide separator for a semi-solid lithium battery according to claim 4, characterized in that: The imidization reagent is composed of acetic anhydride and triethylamine mixed in a molar ratio of 1:

1.

9. The method for preparing a polyimide separator for a semi-solid lithium battery according to claim 4, characterized in that: The molar ratio of the imidization reagent to the total amount of 4,4'-diaminodiphenyl ether is 1:(0.2-0.8).

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